Sectors are generated as cubes, so what you see there is a sector which's star density is much higher than those of its neighbours. As you get closer to the core, such cases can pop up quite often. It's not really a bug, but a way of how Stellar Forge generates systems.
Also, the nebula being central is most likely a coincidence. As far as I know, the procedurally generated nebulae are generated from systems - hence their names. For example, the Eeshorps AA-A h80 started out as a star, then blew up and became a nebula during generation. There are cases where a sector can contain two proc. gen. nebulae: for example, near Jaques there are the Dryooe Prou AA-A h55 and Dryooe Prou AA-A h131 nebulae, both within the Dryooe Prou sector. If I recall correctly, neither of them are in the center of the sector.
